Radio frequency control system



Dec. 8, 1936. A. H. TURNER RADIO FREQUENCY CONTROL SYSTEM Filed Sept. 16, 1950 .(ALFDEN 7 6 0 0%. R w a m R a .2L .IO R mm INVENTOR Alfred H. Turne HIS A TTORNE Y.

Patented Dec. 8, 1936 UNITED STATES PATENT OFFICE RADIO FREQUENCY CONTROL SYSTEM Delaware Application September 16, 1930, Serial No. 482,193

36 Claims.

My invention relates to amplifying systems, and more particularly to control of volume of reproduction in radio receiving circuits employing screen-grid vacuum tubes capable of operating at high amplification.

l-Ieretofore all arrangements for controlling volume in such circuits made according to the teachings of the prior art, and with which I am familiar, have had the following characteristics. Due to the fact that the exceedingly high amplification in screen grid circuits requires the use of volume control means having a great range of continuous variation it has been of necessity common practice to control the volume by means of simultaneously operated separate control devices in difierent parts of the circuit. As an example, a control comprising two variable resistors having a common shaft has been quite common. One resistor has been connected in the antenna circuit or associated with the tuned circuits between tubes to control directly the passage of signal current. The other resistor has been connected to control the amplification as by varying the potentials of the screen-grids or the control grids. In some cases it has been necessary to provide a local-distance switch, an additional manual control, to change the volume in a large step.

The reason for the above design has been that if the input circuit alone is controlled, allowing the amplifier to run at full strength, the local signal is accompanied by just as much amplifier noises as is the distant signal. Furthermore I know of no single control that may be applied to the antenna circuit or other tuned circuit without the introduction of detuning eiiects. On the other hand if the volume is controlled solely by means of adjustment of amplification. as by varying either the screen or the control grid potentials, before the amplification is reduced sufiiciently to control strong local signals a bad form of distortion or cutting-off action is introduced. This is apparently due to the fact that only the peaks of the modulation are amplified.

By combining the two controls and working each within allowable limits the foregoing difliculties have been overcome, but at increased expense, and in many cases with a considerable increase in noise during operation due to the fact that the contact arm in the wire-wound screen-grid control sets up minute electrical disturbances as it passes from turn to turn of the resistor and these disturbances are picked up by the antenna input volume control and amplifled, necessitating shielding between the controls.

It is accordingly an object of my invention to provide a method of and means for controlling volume throughout a great range and capable of continuous volume variation from maximum to minimum by controlling substantially solely the amplification of a screen-grid tube circuit with an absence of the usual distortion caused by working with grid potentials beyond a certain limit.

Another object of my invention is to provide a simplified volume control arrangement comprising a single resistor adapted to cause a simultaneous control action upon a plurality of different species of circuit elements of a screengrid type of circuit.

More specifically stated, it is an object of my invention to provide a volume control arrangement in a high gain screen-grid type of amplifier circuit whereby a single simplified inexpensive wire-wound type resistor of relatively low-resistance, and which needs no taper with my arrangement, varies the bias on the signal control-grids giving adequate control action with absence of distortion.

In accordance with my invention, I control volume by varying the steady potentials of the control-grids and of the screen-grids of one or more tubes simultaneously and in opposite directions thereby permitting use of suificiently high negative bias on the control-grids to minimize amplification without introducing distortion normally resulting from such a negative bias.

Further in accordance with my invention the screen-grids, particularly that of the last radio frequency tube, are floated electrically from the respective anodes or space current supply circuit, each screen-grid being connected through a series high resistance filter to said anode or supply, whereby a signal limiting action is obtained for strong signals and prevents overloading the detector or audio frequency amplifier.

I have found that if in a screen-grid amplifier the positive potential on the screen-grid is increased simultaneously with the negative potential on the control-grid to control ampli fication the above mentioned distortion does not occur, and the control-grid can be used to vary amplification through very wide limits. Furthermore if the screen-grid is energized through a series resistor of high magnitude connected to 91' near the same point of D. C. potential on the power supply circuit as is the anode of the screen-grid tube, when the control grid is made more negative, causing a decreased plate current, the current supplied to the screen-grid through said series resistor likewise decreases, resulting in less drop in said resistor and. hence an increase in voltage on said screen-grid. As this current decreases, approaching zero, the screen-grid potential approaches that of the anode or the potential of its current supply.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawing in which:

Fig. 1 is a circuit diagram of a. screen-grid amplifier embodying my invention, and a ther mionic detector.

Fig. 2 shows a two-stage screen-grid tube amplii-ler embodying modified forms of my invention.

Fig. 3 shows comparative actual operating curves of my improved volume control system.

The system shown in Fig. 1 comprises cascaded screen-grid thermionic amplifier tubes 5, 3, 5 and detector tube i having tuned high frequency input circuits 9, ii, i2, it, connected respectively to signal control-grids id, i2, t l, it.

The plate currents to the screen-grid tubes are supplied, as by means of radio frequency choke coils '25, 21, 29 connected, respectively, to anodes 28, 28 and 3G.

The tubes are of the equi-potential cathode type comprising cathodes 32, 3t and 35 adapted to be energized by alternating current, and derive their space current from a power supply 45 which is ordinarily a rectified alternating current source. Filter condensers il, d9, 5!, 53, 55, 51, 59, El, 63 are of about 0.1 inf. capacity. Each one serves the usual purpose of insuring that the element to which it is connected is at common ground potential for radio frequencies. Series regulating resistors 65, 81, 69, in actual practice of the order of 100,000 ohms, connect the respective screen-grids i8, 20, 22 to the anode supply circuits or spacecurrent supply 45. While it would be simpler to provide a single series resistor to supply all screen-grids connected together and correspondingly reduce the number of condensers, in the case of very high amplification it is preferable that separate resistors be used, as shown, which form with the filter condensers 49, 5!, 55, 51, GI, 63, respectively, very effective individual filter sections in addition to the control functions of said resistors.

Individual self-bias fixed resistors 56, 68 and 10 of the order of 1000 ohms supply the control-grids respectively with the proper operating negative bias. plification it would be simpler .to use but one resistor in common, as in Fig. 2, for all three grids but in case of high amplification I prefer separate ones for well known reasons. In some cases I prefer to make the bias resistor 66 of the first tube of different value from that of resistors 68 and 10.

Volume control resistor H is in series with the self bias resistors 66, 68 and I0 and serves to add more bias resistance, making the oath- Here again, for lower am-,

odes more positive with respect to ground, or in other words increasing the negative bias on the control-grids. Resistor 13 is a "bleeder" resistor of the order of 20,000 ohms which is virtually in shunt with the space current paths of the tubes to cause more current through resistor H, resulting in the desired amount of IR drop through the volume control resistor with a smaller value of resistor. Practically, a 2000 ohm resistor H is more rugged and simpler than a much higher value resistor. The values of bleeder resistor and volume control II are relatively so chosen that the negative bias on the control grids rises to about thirty volts when the volume control is in position of minimum volume. It would be possible to make control resistor ll fixed and vary the current therethrough as by variation of bleeder resistor 13, or by means of an automatic volume control tube.

It has been found that the above-mentioned cutting-oil action occurs when an excessive signal is applied to the grid of a tube having too great a negative bias on the control-grid. This condition occurs when individual screen-grid filter resistors are used and when the amplification of one of the last stages is reduced at a greater rate than that of the preceding stages or a leading-stage, which action may occur in one of said last stages by the use of a tube having certain non-uniform characteristics with relation to the other tubes in the amplifier. Hence by insuring reduction of the amplification of the first stage, or said preceding stages, at a relatively greater rate than that of at least one of said last stages during reduction of volume, the system is made independent of such nonuniformity of tube characteristics as might occur during use. Otherwise it would be necessary to interchange the tubes in such manner as to achieve this result. In actual practice I have found it desirable to modify the floating action of screen-grid of the first tube i to effect such an increased rate of change in amplication, as by means of a high resistor of the order of 100,000 ohms connected from the screengrid to ground. Resistors 65 and i5 constitute a high resistance potentiometer across the power supply source 45. An additional advantage of this modification is a reduction 'of a tendency to a type of interference known as cross-modulation. It is possible to obtain somewhat the same result by increasing the size of the first bias resistor 66 as indicated in Fig. 2, but the foregoing method is much preferred. Connecting the screen-grids together, as in Fig. 2, and using a common screen-grid filter series resistor prevents the above distortion due to non-uniform tube characteristics.

The resistor and condenser values above mentioned are not critical and may be altered considerably without changing performance. The relative potentials may likewise be altered considerably; I have found that greater range of amplification control free of above distortion is obtainable by permitting the screen-grids potentials to exceed the anode voltage when operating with certain higher values of negative controlgrid bias.

Referring to the characteristic curves in Fig. 3, curve B shows how the plate current varies as the negative control grid bias is varied by means of the usual volume control in a radio circuit. The plate voltage was 220 and the screen grid voltage was held constant at V.,

automatically as the negative control grid bias voltage is increased or the volume control retarded, as in the case of my invention. As in the case of curve B, the plate voltage'was 220 V.

Curves C, D, and E represent dynamic characteristics at diflerent operating points of retarded olume.

By reason of my invention, in' actual practice the range of undistorted volume-control, in connection with ordinary screen grid tubes, has been extended very considerably, aswill be seen from a comparison of curves A and. B. The distortion at the useful minimum limit of mutual conductance is a function of the rate of change of curvature of these performance-curves, being least for the smallest rate of change of curvature. Curve A illustrates how tlie'plate current cut-ofi point has been substantially extended with increased grid bias, showing the effect of the regulating action of the screen grid, connected in accordance with my invention, to compensate for the tendency of grid bias greater than normal to cause distortion. 1

In the modification of Fig. 2 a similar simultaneous action is obtained by means of .a dual volume control. Here a sliding arm 11 is provided to pick oil the desired screen-grid potential from the bleeder resistor 73, and the contact arms 16 and I1 are mechanically coupled. By proper tapering of either or both resistors the relative potential changes on control-grids and screen-grids may be governed. This flexibility of design might be an advantage over the automatic action obtained in the first circuit in certain critical cases but the first circuit species is preferred in general practice. In Fig. 2 I have shown a screen-grid detector til of the grid detection type, although it maybe of the plate detection type, and have illustrated the application of the above screen-grid series resistor filter comprising resistor ti and condensers B3 and tit, the resistor being connected to the power-supply circuit adjacent the low potential end of primary of transformer id. one advantage of this filter arrangement in both the detector and amplifier is that the wiring is simplified in that it is not necessary to run a separate lead from the screen-grid to the power supply. If plate detection is used, giving smaller plate and screen-grid currents, it is possible to greatly increase-the size of the series resistor ill, say to one or more megohms, and thereby obtain improved filtering action.

Although not essential, nor in most cases desirahle, the antenna input signal may likewise be controlled simultaneously with. the amplification by the simple expedient of connecting the antenna 36 or input circuit as represented by coil M to the lower end of volume control resistor ii. This connection is illustrated in Fig. 2 by lead ill. With such an addition the single volume control ll would in Fig. l perform three functions. 7

In some cases it is desired to make one of the radio frequency tubes, preferably that of the last radio frequency stage limit the amplitude of the radio frequency signal and thus prevent distortion caused by overloading the detector or audio frequency amplifier in the case of excessively strong signals. I have found that the screenaction. The application of a radio frequency potential upon the control-grid of amplifier tube 5 tends to decrease the effective control-grid potential, thereby decreasing the screen grid cathode resistance of the .tube. However, in- .creased screen-grid current is opposed by the high magnitude of resistor 69. Consequently the screen-grid potential is automatically lowered and the amplification, of the tube reduced in response to an excessive signal strength.

Although I have illustrated my invention in.

its application to radio frequency stages it is obvious to anyone skilled? in the art that the same invention could be used in an audio frequency amplifier using screen-grid type tubes. The invention may have utility in connection with other grid arrangements e. g. "Pentodes" wh ch have three grids. Furthermore-the invention has been illustrated only in connection with manual control of amplificationbut it is likewise obvious that the circuit of Fig-'1 would i have equal utility when usedin conjunction with type shown by- Afiel Patent #1,511,015,. where aautomatic volume control e. g. broadly ;o f the volume control tube, in response to a change in rectified carrier current level, causes varying amounts of current to fiow through a resistor connected in circuit with the grid electrodes,

thereby changing the voltage drop and the corresponding grid bias.

For purposes of illustration I have shown the resistor 13 in Fig. 2 connected to a higher point of potential than the anodes. permitting the screen-grids to be made more positive than the anodes, thus allowing an increased negative bias on the control grids for greater range of volume control. The same principle may be applied to the automatic action of the floating screen-grids of Fig. 1. Although the screen-grid resistors have in Fig.1 been shown as connected respectively to apoint in the anode circuit it is within the scope of my invention to connect some or all of the screen-grids individually, or together thru a common series high resistor, to a point of potential on supply source it higher thanthe potential of the anodes. When connecting to. such a higher potential the series screen-grid resistor or resistors are made considerably'higher in value.

I do not wish to be restricted to the specific structural details, arrangement of parts or circuit connections herein set forth, as various other modifications thereof may beeffected without departing from the sp rit and scope of my invention. I desire therefore that only such limitations shall be imposed as are indicated in the appended claims. j

What I claim as new is:

l. The combination in an amplifying systememploying a multi-grid thermion c tube having a cathode, a first grid, a second grid and an anode, of amplification control meansfor varying the potential on said first grid in a negative direction to decrease amplification and means. for simultaneously varying the potential on said second grid in a positive direction to minimize becomes excessive.

2. The combination in an amplifying system employing a multi-grid thermionic tube comprising a cathode, a normally negatively biased grid and a normally positively biased grid, both with respect to said cathode, of volume control means for changing the bias of said negatively biased grid in a negative direction to decrease amplification, and means responsive to a change in said bias for increasing the bias on said positively biased 811d.

3. The combination in an amplifying system employing a screen-grid thermionic tube, of amplification control means for changing the control grid bias 01' said tube in a negative direction to decrease amplification, and means for simultaneously changing the screen-grid bias oi said tube in a positive direction, tending to increase-amplificatimbut in an extent insuificient to offset the decrease in amplification effected by said control grid bias change.

4. The combination in an amplifying system employing a screen-grid thermionic tube, of control means for changing the control grid bias of said tube in a negative direction to decrease amplification means for simultaneously changing the screen-grid bias of said tube in a positive direction to increase amplification, whereby a desired change in the plate current-grid bias characteristic curve is eifected independently of amplification.

5. The combination in an amplifying system employing a screen-grid thermionic tube having a negatively biased control grid and a positively biased screen grid, of a volume control circuit comprising means for varying the negative bias on the control-grid of said tube and means responsive to a change in said bias for simultaneously varying the potential on the screengrid of said tube in an opposite sense to prevent distortion.

6. The combination in an amplifying system employing a screen-grid thermionic tube energized from a space current source, amplification control means for varying the control-grid bias of said tube and compensating means responsive to a change in said bias for simultaneously varying the screen-grid potential of said tube in an opposite sense comprising a high resistance element serially connecting said screen grid to said current source.

'7. The combination in an amplifying system employing a screen-grid thermionic tube energized from a power supply source, manually operated amplification control means for varying the control-grid bias of said tube and a resistance capacity filter for supplying direct current to the screen-grid of said tube comprising a high resistance element of the order of 100,000 ohms connected in series between said screengrid and said supply source.

8. The combination in an amplifying system employing a screen-grid thermionic tube energized from a space-current source, means for varying the control-grid bias of said tube comprising a volume control resistor serially disposed in the space current path of said tube, an auxiliary resistor effectively in shunt with said space current path to increase the available voltage drop through said volume control resistor and a compensating resistor 01' high fixed magnitude connecting the screen-grid of said tube to said current source whereby the voltage drop through said volume control resistor decreases as the control-grid negative bias increases.

9. The combination in an amplifying system employing screen-grid thermionic tubes in cascade and energized from a space-current source, means for negatively biasing the control-grids of said tubes comprising a fixed resistor connected between the cathode of each tube and a common point and a volume control resistor connected between said point and the negative end of said space-current source, an auxiliary resistor connected between a positive point on said space-current source and said common point to increase the current through said volume control resistor, means for varying the voltage drop across said control resistor to vary said negative grid bias, and means for simultaneously varying the screen-grid potentials of said tubes.

10. The combination in an amplifying system employing screen-grid thermionic tubes in cascade and energized from a space-current source, an amplification control circuit for varying the negative bias of the control-grids of said tubes, a resistance capacity filter for supplying current to each screen-grid from said source com-' prising a series high resistance compensating element connected between said source for varying the respective screen-grid potentials in response to said variation of control-grid bias and in an opposite algebraic sense thereto and means for modifying the compensating action of the series resistor element connected to the screengrid of the first tube of said cascade comprising a resistor connected from said grid to the low potential end of said source of space current.

11. The combination in an amplifying system employing screen-grid thermionic tubes in cascade and energized from a space-current source,

I an amplification control circuit for increasing the negative bias of the controbgrids of said tubes to decrease amplification and means simultaneously operated for causing the potential of the screen-grids of one or more of said tubes to gradually increase in a positive direction and substantially reach or exceed the potential of the anodes.

12. The combination in an amplifying system employing screen-grid thermionic tubes in cascade and energized from a space-current and bias potential source, an amplification control circuit for increasing the negative bias of the control-grids of said tubes to reduce amplification, means simultaneously operated for causing the potential of the screen-grids of one or more of said tubes to approach the potential of the anodes and means for causing the reduction of amplification of the first of said tubes to occur at a greater rate than that of any of the tubes following.

13. In an amplifying system employing screengrid thermionic tubes in cascade, energized by a source of space-current and bias potentials, the method of controlling amplification which comprises increasing the negative bias on the control-grid of one of said tubes to produce a decrease in amplification and simultaneously increasing the positive screen-grid bias on said tone of said tubes tending to increase amplificaion.

14. The combination tem employing screen-grid thermionic tubes in cascade, of a signal input circuit, a source of bias potentials, a volume control resistor for varying the negative control-grid bias on said tubes, means for connecting the input circuit to said resistor for simultaneous variation of the with an amplifying syinput signal strength and means for simultaneously varying the screen-grid potential of one or more of said tubes oppositely with respect to said control-grid variation.

15. The combination in a signal receiving system employing a screen-grid thermionic tube energized by a space current and bias potential supply source, of a signal input circuit, a volume control resistor having a movable contact element for varying the negative bias on the control grid of said tube, a second resistor having a movable contact element for varying the positive potential on the screen-grid of said tube, means for mechanically connecting said elements for simultaneous adjustment and means for connecting the input circuit to one of said resistors for simultaneous variation of the input signal.

16. The combination in a signal receiving system associated with an antenna input and a screen-grid thermionic tube energized by a space current supply source having its negative side at ground potential, a volume control resistor having a movable contact element, one end of said resistor being connected to the cathode, the other end thereof to said antenna input and said contact element to ground and means for simultaneously varying the screengrid potential of said tube. v

17. In an amplifying system employing screengrid thermionic tubes in cascade, energized by a source of bias potentials and space-current, the method of reducing amplification .which comprises increasing the negative bias on the control-grids of a plurality of said tubes, in-

creasing the positive screen-grid bias of some of said tubes, and causing a reduction in ampliflcation of one of the first of said tubes in cascade to occur at a greater rate than that of any of the tubes following.

18.1n an amplifying system, a thermionic tube having a cathode, a control grid, a screen grid and an anode, a potential source the negative terminal of which is connected to ground, a resistor one end of which is connected to the cathode and the other end of which is connected to the control grid, a second resistor one end of which is connected to the positive terminal of the-source and the other end of which is connected to the cathode, a plurality of contact elements, one for each resistor, the contact element associated with the first mentioned resistor being connected to ground and the contact element associated with the second named resistor being connected to the screen grid and means for simultaneously adjusting said contact elements toward and away from the ends of the several resistors connected to the cathode, whereby simultaneous change in control grid biasing potential and screen grid potential may be had.

19. The invention set forth in claim 18 further characterized in that the thermionic tube is provided with an antenna input circuit disposed in shunt relation to the first named resistor and the grounded movable element whereby input potentials may also be adjusted by movement of the contact elements.

20. The invention set forth in claim 18 further characterized in that a third self-bias resistor is included in the connection to the cathode from the two first-mentioned resistors.

21. In the operation of an amplifier circuit of the type including an audion having a. plurality of electrodes, and means for establishing diflerent direct current potentials upon the several electrodes, the method of controlling volume output which comprises primarily adlusting the direct current potential applied to an input electrode in a direction and to an extent tending to produce a cut-off eflect, and simultaneously adjusting the potential on another electrode to compensate against said tendency.

22. The invention as set forth in the above claim 21, wherein the potentials upon said two of said electrodes are varied at diflerent rates to control the amplification.

23. In the operation of an amplifier circuit of the type including an audion having a plurality of grid elements, and means for establishing different direct current potentials upon the several elements, the method of controlling the amplification which comprises simultaneously adjusting the direct current potentials applied to at least two of said elements, wherein the potentials upon said two of said elements are varied in opposite sense.

24. In the operation of an amplifier circuit of the type including an audion having a plurality of grid elements, and means for establishing different direct current potentials upon the several elements, the method of controlling the amplification which comprises simultaneously adjusting the direct current potentials applied to at least two of said elements, wherein the potentials upon said two of said elements are varied in opposite sense and at different rates.

25. In the operation of an amplifier tube of the type having a cathode, an anode, a control grid and a second grid, the method of controlling volume output which comprises simultaneously adjusting the direct current potentials between the respective grids and the cathode and adjustably impressing signal potentials on one of said grids in accordance with the change in direct current potentials on one of said grids.

26. In an amplifier circuit, an amplifier tube having a cathode, an anode, a control grid and a second grid, means for establishing direct current potentials between said cathode and each of the other tube electrodes, means for adjustably impressing a signal voltage between cathode and control grid, and means for simultaneously adjusting the direct current potentials upon said control grid and upon another tube electrode.

27. An amplifier circuit comprising an electron discharge tube provided with a cathode, anode, control grid, and auxiliary grid, means for operating said grids at different signal potentials, and additional means for simultaneously effecting variation of the direct current potentials of the grids at difierent rates and for varying impressed signal potentials on one 01 said grids.

28. An amplifier circuit comprising an electron discharge tube provided with a cathode, anode, control grid, and auxiliary grid, means for maintaining said grids at diiferent signal potentials, and additional means for simultaneously varying the direct current potentials of the grids in op posite sense and at different rates.

29. In a thermionic amplifier having input and output circuits, the method of controlling volume output which comprises adjusting an operating direct current bias on an input electrode of said amplifier in a direction and to an extent tending to cut ofi space current, and simultaneously compensating in the output circult to substantially extend the point of cut-off of said amplifier.

30. The combination in an amplifying system employing a plurality of screen grid thermionic tubes energized from a space current source, a volume control circuit comprising means for varying the negative bias on the control grids of said tubes, and means common to said screen grids and connected to said source for applying a direct current potential to said screen grids, said common means being responsive to a change in control grid bias for simultaneously varying the potential on the screen grids to prevent distortion caused by an increase in control grid bias beyond normal.

31. The invention as set forth in claim 6 characterized by a plurality of screen grid tubes in said system, the screen grids of which are connected through said high resistance element in common to the several screen grids.

32. In a modulated carrier receiving system employing screen grid tubes in cascade, a space current supply source for energizing said tubes, means providing operating negative bias on the control grids of a plurality of said tubes, amplification control means for increasing the negative grid biases of said plurality of tubes for reducing amplification thereof, and regulating means in circuit with a screen grid of at least one of amplification controlled tubes for causing the reduction in amplification thereof to occur at a slower rate than that of at least one other of said controlled tubes.

33. The invention as set forth in claim 32 characterized by the fact that the amplification reduction of a leading one of said controlled tubes occurs at a greater rate than that of a following tube that is provided with said regulating means.

34. In a radio receiving system employing screen grid tubes in cascade, a space current supply source for energizing said tubes, means providing operating negative bias on the control means grids of a plurality of said tubes, amplification control means for increasing the negative grid biases of said plurality of tubes for reducing amplification thereof, and means for simultaneously increasing the applied positive screen grid potential of at least one of said tubes controlled by said amplification control means, for efi'ecting a slower rate of amplification reduction than that of at least one other of said controlled tubes.

35. In an amplifying system employing screen grid thermionic tubes in cascade and supplied with operating bias and space current, the method of reducing amplification which comprises increasing the negative bias on the control grids of a plurality of said tubes, increasing the positive screen grid bias of at least one of said plurality of tubes, and causing a reduction in amplification of at least one of said plurality of said tubes in cascade to occur at a greater rate than that of at least one of the succeeding tubes.

36. In a radio receiving system employing screen grid tubes in cascade, a space current supply source for energizing said tubes, means providing operating negative bias on the control grids of a plurality of said tubes, amplification control means for increasing the negative grid biases of said plurality of tubes comprising a bias resistor connected in common circuit relation with said plurality of tubes to reduce amplification thereof, means for simultaneously increasing the applied positive screen grid potential of at least one of said plurality of tubes comprising a high resistance element connected in series between said space current source and said screen grid, and means for supplying oper- ALFRED H. TURNER. 

